Recent demand for high integration and speed-up of electric circuits requires that a semiconductor apparatus is manufactured in an increasingly refined manner. However, since the voltage used in a semiconductor apparatus does not vary in accordance with the scale of miniaturization, a problem with reliability inevitably surfaces because of the electric field concentration in a transistor of a very refined structure. Accordingly, a power supply by means of a so-called voltage drop circuit, for supplying an externally applied voltage to an internal circuit after dropping the same voltage, is desired in order to ensure reliability without changing the voltage used in a semiconductor apparatus.
Conventionally, a power circuit of this type comprises a variable resistance element, a reference voltage generating source, a comparator, and a resistance element driver, for the purpose of supplying a power supply voltage to an internal circuit.
A variable resistance element changes its value of resistance in response to a signal fed into its control input terminal and is inserted in the line which supplies power to an internal load circuit. A reference voltage generating source generates a reference voltage that serves as a reference for a voltage applied to the internal load circuit. A comparator compares the reference voltage obtained from the reference voltage generating source with an applied voltage actually applied to the internal circuit. A resistance element driver drives the variable resistance element in accordance with an output signal from the comparator, thereby maintaining the applied voltage applied to the internal circuit at the same level on the basis of the variation of the value of resistance.
In such a power circuit, when a voltage V.sub.DD (3V, normally) varies due to such causes as the variation of an externally supplied power supply voltage V.sub.CC (5V, normally) or the operation of the internal circuit, a negative feedback is applied to the variable resistance element so as to cancel the variation, with the result that the applied voltage V.sub.DD of the internal circuit is maintained at approximately the same level.
However, since it is impossible in such a conventional power circuit to secure a large idling current in the variable resistance element driver due to the requirement of reducing power consumption, there is a disadvantage in that a bad follow-up characteristic results with respect to an instantaneous variation (noise) of the internal circuit applied voltage V.sub.DD, caused by the operation of the internal circuit, for example.
The present invention is designed to eliminate the above disadvantage, and the object thereof is to improve the follow-up characteristic of such a power circuit with respect to an instantaneous variation (noise) of the internal circuit applied voltage V.sub.DD without increasing the idling current of the variable resistance driver.